The Curtiss O-40 Raven was an American observation aircraft of the 1930s which was built and used in small numbers. A single example of the YO-40, a single-engined Sesquiplane with a retractable undercarriage was built, followed by four examples of a modified monoplane version, the O-40B, which remained in use until 1939.

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In 1931, in response to a United States Army Air Corps requirement for a new observation aircraft, Curtiss designed the Model 62, a single-engined biplane with the lower wing much smaller than the upper, known as a sesquiplane (i.e. "one-and-a-half wings"), while the outer panels of the upper wings were swept back to avoid centre-of-gravity problems. It was of all-metal construction, with a monocoque fuselage and had a retractable tailwheel undercarriage with inwards retracting mainwheels, and was powered by a Wright Cycloneradial engine. The crew of two sat in tandem in open cockpits.[1][2][3]

A prototype, designated YO-40, flew in February 1932. Although it crashed in May that year, it was rebuilt with stronger wings and an enclosed cockpit as the YO-40A. A further four YO-40As were ordered, but they were redesigned as monoplanes by removing the lower wing, the resultant aircraft being designated Y1O-40B.[2][4]

The four Y1O-40Bs were delivered in June 1933, and after service tests, were redesignated O-40B,[5] being operated by the 1st Observation Squadron of the USAAC.[6] While the aircraft's performance and manoeuvrability were good, the Air Corps was disappointed with the cockpit arrangements and the low fuel capacity,[2] and no more orders followed. The last O-40B was withdrawn from use in 1939.[5]